Power dividers and combiners are useful in a wide variety of circuits. Specific applications include combining multiple power amplifier stages in order to achieve a desired high power output. Since most solid state power devices, such as MESFETs, PHEMTs, and bipolar transistors have low input and output impedances, successive impedance transformations are often necessary to achieve 50 ohm input and output impedance levels.
There are several technologies which currently provide power dividing/combining, including radial combiners, split lines, and branch line combiners. While these power combining/dividing methods and apparatus may be suitable for many applications, they do not provide for power combining/dividing over a broadband of frequencies with good isolation between the combined signals while simultaneously achieving a wide range of impedance transformations in a compact size. For example, radial combiners, typically machined out of metal, tend to be large structures and not well suited to size-critical applications. Simple split lines divide or combine power simply, but offer no isolation between ports. Branch line couplers are reactive, and have no resistors to dissipate out-of-phase energy.
A power combiner/divider known as the Wilkinson power divider offers binary dividing/combining (i.e., successive divisions or multiplications by two). The Wilkinson power divider/combiner is limited in that the divisions/multiplications are always by a factor of 2 and the input and output impedances are all equal to a characteristic impedance Z.sub.o. The Wilkinson design does not facilitate the use of different input and output impedances regardless of whether it is used as a combiner or a divider. Since the Wilkinson power divider/combiner uses quarter-wavelength lines in each division/multiplication and is binary, each division/multiplication past the first requires space for the additional quarter-wavelength lines. Moreover, the Wilkinson power divider/combiner does not offer N-way combining, low insertion loss, or broad bandwidth response.
There is a need for power combining/dividing and impedance transformation functions over a broad band of frequencies with good isolation between the combined signals, while simultaneously achieving a wide range of possible impedance transformations using a method and apparatus suitable for use in microstrip technologies, including microwave monolithic integrated circuits (MMICs). To be cost effective, these functions must be accomplished without requiring a great deal of surface area on a semiconductor die.